Credit for changing this perception goes to Pierre de Fermat (1601–65), a French magistrate with time on his hands and a passion for numbers. Although he published little, Fermat posed the questions and identified the issues that have shaped number theory ever since. Here are a few examples:

In 1640 he stated what is known as Fermat’s little theorem—namely, that if p is prime and a is any whole number, then p divides evenly into ap − a. Thus, if p = 7 and a = 12, the far-from-obvious conclusion is that 7 is a divisor of 127 − 12 = 35,831,796. This theorem is one of the great tools of modern number theory.

Fermat investigated the two types of odd primes: those that are one more than a multiple of 4 and those that are one less. These are designated as the 4k + 1 primes and the 4k − 1 primes, respectively. Among the former are 5 = 4 × 1 + 1 and 97 = 4 × 24 + 1; among the latter are 3 = 4 × 1 − 1 and 79 = 4 × 20 − 1. Fermat asserted that any prime of the form 4k + 1 can be written as the sum of two squares in one and only one way, whereas a prime of the form 4k − 1 cannot be written as the sum of two squares in any manner whatever. Thus, 5 = 22 + 12 and 97 = 92 + 42, and these have no alternative decompositions into sums of squares. On the other hand, 3 and 79 cannot be so decomposed. This dichotomy among primes ranks as one of the landmarks of number theory.

In 1638 Fermat asserted that every whole number can be expressed as the sum of four or fewer squares. He claimed to have a proof but did not share it.

Fermat stated that there cannot be a right triangle with sides of integer length whose area is a perfect square. This amounts to saying that there do not exist integers x, y, z, and w such that x2 + y2 = z2 (the Pythagorean relationship) and that w2 = 1/2(base) (height) = xy/2.

Uncharacteristically, Fermat provided a proof of this last result. He used a technique called infinite descent that was ideal for demonstrating impossibility. The logical strategy assumes that there are whole numbers satisfying the condition in question and then generates smaller whole numbers satisfying it as well. Reapplying the argument over and over, Fermat produced an endless sequence of decreasing whole numbers. But this is impossible, for any set of positive integers must contain a smallest member. By this contradiction, Fermat concluded that no such numbers can exist in the first place.

Two other assertions of Fermat should be mentioned. One was that any number of the form 22n + 1 must be prime. He was correct if n = 0, 1, 2, 3, and 4, for the formula yields primes 220 + 1 = 3, 221 + 1 = 5, 222 + 1 = 17, 223 + 1 = 257, and 224 + 1 = 65,537. These are now called Fermat primes. Unfortunately for his reputation, the next such number 225 + 1 = 232 + 1 = 4,294,967,297 is not a prime (more about that later). Even Fermat was not invincible.

The second assertion is one of the most famous statements from the history of mathematics. While reading Diophantus’s Arithmetica, Fermat wrote in the book’s margin: “To divide a cube into two cubes, a fourth power, or in general any power whatever into two powers of the same denomination above the second is impossible.” He added that “I have assuredly found an admirable proof of this, but the margin is too narrow to contain it.”

In symbols, he was claiming that if n > 2, there are no whole numbers x, y, z such that xn + yn = zn, a statement that came to be known as Fermat’s last theorem. For three and a half centuries, it defeated all who attacked it, earning a reputation as the most famous unsolved problem in mathematics.

Despite Fermat’s genius, number theory still was relatively neglected. His reluctance to supply proofs was partly to blame, but perhaps more detrimental was the appearance of the calculus in the last decades of the 17th century. Calculus is the most useful mathematical tool of all, and scholars eagerly applied its ideas to a range of real-world problems. By contrast, number theory seemed too “pure,” too divorced from the concerns of physicists, astronomers, and engineers.

Credit for bringing number theory into the mainstream, for finally realizing Fermat’s dream, is due to the 18th century’s dominant mathematical figure, the Swiss Leonhard Euler (1707–83). Euler was the most prolific mathematician ever—and one of the most influential—and when he turned his attention to number theory, the subject could no longer be ignored.

Initially, Euler shared the widespread indifference of his colleagues, but he was in correspondence with Christian Goldbach (1690–1764), a number theory enthusiast acquainted with Fermat’s work. Like an insistent salesman, Goldbach tried to interest Euler in the theory of numbers, and eventually his insistence paid off.

It was a letter of December 1, 1729, in which Goldbach asked Euler, “Is Fermat’s observation known to you, that all numbers 22n + 1 are primes?” This caught Euler’s attention. Indeed, he showed that Fermat’s assertion was wrong by splitting the number 225 + 1 into the product of 641 and 6,700,417.

Through the next five decades, Euler published over a thousand pages of research on number theory, much of it furnishing proofs of Fermat’s assertions. In 1736 he proved Fermat’s little theorem (cited above). By midcentury he had established Fermat’s theorem that primes of the form 4k + 1 can be uniquely expressed as the sum of two squares. He later took up the matter of perfect numbers, demonstrating that any even perfect number must assume the form discovered by Euclid 20 centuries earlier (see above). And when he turned his attention to amicable numbers—of which, by this time, only three pairs were known—Euler vastly increased the world’s supply by finding 58 new ones!

Of course, even Euler could not solve every problem. He gave proofs, or near-proofs, of Fermat’s last theorem for exponents n = 3 and n = 4 but despaired of finding a general solution. And he was completely stumped by Goldbach’s assertion that any even number greater than 2 can be written as the sum of two primes. Euler endorsed the result—today known as the Goldbach conjecture—but acknowledged his inability to prove it.

Euler gave number theory a mathematical legitimacy, and thereafter progress was rapid. In 1770, for instance, Joseph-Louis Lagrange (1736–1813) proved Fermat’s assertion that every whole number can be written as the sum of four or fewer squares. Soon thereafter, he established a beautiful result known as Wilson’s theorem: p is prime if and only if p divides evenly into[(p−1) × (p−2) × ⋯ × 3 × 2 × 1] + 1.

Number theory in the 19th century

Disquisitiones Arithmeticae

Of immense significance was the 1801 publication of Disquisitiones Arithmeticae by Carl Friedrich Gauss (1777–1855). This became, in a sense, the holy writ of number theory. In it Gauss organized and summarized much of the work of his predecessors before moving boldly to the frontier of research. Observing that the problem of resolving composite numbers into prime factors is “one of the most important and useful in arithmetic,” Gauss provided the first modern proof of the unique factorization theorem. He also gave the first proof of the law of quadratic reciprocity, a deep result previously glimpsed by Euler. To expedite his work, Gauss introduced the idea of congruence among numbers—i.e., he defined a and b to be congruent modulo m (written a ≡ b mod m) if m divides evenly into the difference a − b. For instance, 39 ≡ 4 mod 7. This innovation, when combined with results like Fermat’s little theorem, has become an indispensable fixture of number theory.

From classical to analytic number theory

Inspired by Gauss, other 19th-century mathematicians took up the challenge. Sophie Germain (1776–1831), who once stated, “I have never ceased thinking about the theory of numbers,” made important contributions to Fermat’s last theorem, and Adrien-Marie Legendre (1752–1833) and Peter Gustav Lejeune Dirichlet (1805–59) confirmed the theorem for n = 5—i.e., they showed that the sum of two fifth powers cannot be a fifth power. In 1847 Ernst Kummer (1810–93) went further, demonstrating that Fermat’s last theorem was true for a large class of exponents; unfortunately, he could not rule out the possibility that it was false for a large class of exponents, so the problem remained unresolved.

The same Dirichlet (who reportedly kept a copy of Gauss’s Disquisitiones Arithmeticae by his bedside for evening reading) made a profound contribution by proving that, if a and b have no common factor, then the arithmetic progression a, a + b, a + 2b, a + 3b, … must contain infinitely many primes. Among other things, this established that there are infinitely many 4k + 1 primes and infinitely many 4k − 1 primes as well. But what made this theorem so exceptional was Dirichlet’s method of proof: he employed the techniques of calculus to establish a result in number theory. This surprising but ingenious strategy marked the beginning of a new branch of the subject: analytic number theory.